1. Field of the Invention
The present invention relates to copper foil for fine pattern printed circuits and a method for production of the same.
2. Description of the Related Art
Normally, electrodeposited copper foil is produced by two processes. The first process is for making the foil by an electrodeposited foil-making system. The second process is for roughening treating the surface to improve the bondability and performing various other surface treatment by a surface treatment system so as to produce the type of copper foil suitable for a printed circuit board. FIG. 1 shows the first process in the production of electrodeposited copper foil. The electrodeposited foil-making system is comprised of a rotating drum-like cathode (made of stainless steel or titanium) 2 and an anode (made of Pb or DSA) 1 arranged concentrically and cylindrically with respect to the cathode 2. A copper plating solution 3 is passed between the cathode 2 and the anode 1 and a current is passed across the electrodes so as to cause copper to deposit to a predetermined thickness on the cathode 2. This is then peeled off to obtain a copper foil 4. This copper foil 4 will be called the “untreated copper foil” in this specification. The untreated copper foil is then given the properties required for a copper-clad laminate by the second process where, as shown in FIG. 2, it is continuously treated on its surface electrochemically or chemically. FIG. 2 shows a surface treatment system for treating the surface of the untreated copper foil. This passes the untreated copper foil 4 continuously through an electroplating tank filled with an electrolytic solution 5 and an electroplating tank filled with an electrolytic solution 6 and treats its surface using the electrodes 7 as the anode and the copper foil itself as the cathode and thereby produces surface-treated copper foil 8. The copper foil surface treated in this way will be called the “surface-treated copper foil” in this specification. The surface-treated copper foil is used for a printed circuit board.
The surface treatment method of the untreated copper foil enables the copper foil to be strongly bonded with a resin board or enables the electrical properties, etching properties, heat resistance, or chemical resistance required for a printed circuit board to be satisfied by roughening treating the surface of the copper foil to be bonded with the resin board and by plating the thus roughening treated surface with zinc, nickel, etc. or further treating the zinc, nickel, or otherwise plated surface by chromate, a silane coupling agent, etc. As one example, the method has been disclosed of using the copper foil as a cathode in an acidic copper plating bath and performing so-called “burnt plating” near the limit current density so as to roughen the surface of the copper foil to be bonded (for example, see Japanese Examined Patent Publication (Kokoku) No. 40-15327). Further, the method has been disclosed of roughening treating the surface of copper foil to be bonded and covering the surface of the roughening treated side having a plurality of fine projections with an smooth thin layer of copper plating (so-called “encapsulation layer” so as to stably fix the plurality of fine projections of the roughening treated side to the copper foil (for example, see specification of U.S. Pat. No. 3,293,109). This series of treatments is called “roughening treating” in this specification.
A printed circuit board using such a surface-treated copper foil is usually produced in the following way. First, the surface of an electrically insulating substrate comprised of a glass epoxy resin, a polyimide resin, etc. is covered with the surface-treated copper foil for surface circuit formation, then these are heated and pressed to produce a copper-clad laminate. Next, the copper-clad laminate is formed with through holes and the through holes plated, then the copper foil at the surface of the copper-clad laminate is etched to form circuit patterns having the desired line widths and desired space widths. Finally, the solder resist is formed and other finishing performed. The copper foil used at this time has a rough side as the surface of the side to be hot press bonded to the substrate. This rough side exhibits an anchoring effect to the substrate and thereby improves the bond strength between the substrate and copper foil and ensures reliability of the printed circuit board.
Further, recently, resin-coated copper foil comprised of copper foil with the rough side covered by an adhesive resin such as an epoxy resin in advance and using that adhesive resin as a semicured state (B-stage) insulating resin layer has been used as copper foil for surface circuit formation. The insulating resin layer side has been hot press bonded to the substrate to produce a printed circuit board, in particular a builtup printed circuit board. Further, to deal with the increasing higher integration of various electronic devices, such builtup printed circuit boards have been required to offer higher densities of circuit patterns as well. So-called “fine pattern” printed circuit boards with circuits of fine line widths and space widths have begun to be demanded. For example, in the case of printed circuit boards used for semiconductor packages, printed circuit boards having high density superfine circuits of line widths and space widths of around 15 μm are being demanded.
If using copper foil with a large roughness of the surface as copper foil for forming such a printed circuit board, the time required for etching down to the surface of the substrate will become longer. As a result, as shown in FIG. 3, the perpendicularity of the side walls in the circuit patterns of the copper foil A clad with the substrate B will be ruined and the etching factor (Ef), as expressed by the following formula:Ef=2T/(Wb−Wt)(where, T is the thickness of the copper foil, Wb is the bottom width of the circuit patterns formed, and Wt is the top width of the circuit patterns formed), will become small. This problem does not become that severe when the line widths in the circuit patterns formed is large, but in the case of fine line width circuit patterns, it could lead to missing conductors. To deal with the demands for fine patterns, one of the important factors having a large effect on the etchability in the performance of copper foil is the roughness of the surface. In particular, the roughness of the surface which is roughening treated for bonding with a resin substrate has a large effect. The factors influencing the roughness of copper foil can be largely classified into two. One is the surface roughness of the rough side of the untreated copper foil, while the other is the manner of deposition of the granular metal deposited by the roughening treating (plating). If the surface roughness of the rough side of the untreated copper foil is large, the roughness of the surface of the copper foil after the roughening treating will also become large. Further, if the amount of deposition of the granular metal is large, in general the roughness of the surface of the copper foil after roughening treating will become large. The roughness of the rough side of the untreated copper foil is largely determined by the electrolytic conditions when causing deposition of copper on the drum-like cathode when producing copper foil electrolytically, in particular the additives used in the electrolytic solution. Further, the shapes of the grains and the method of deposition are largely affected by the composition of the copper plating solution and plating conditions of the “burnt plating” forming the roughening treating.
In general, when producing a so-called “shiny side”, the surface at the side contacting the drum is relatively smooth, but the opposite side, that is, the surface in contact with the copper plating solution, has rough surfaces. Therefore, as an experiment for smoothing the rough side, for example the method of producing electroplated copper foil by adding thiourea or other active sulfur to the copper plating solution has been disclosed (for example, see specification of U.S. Pat. No. 5,171,417). The rough side of the untreated copper foil produced by this method, however, does indeed have a small Rz value of the rough surfaces (note that the “surface roughness Rz” spoken of here is the 10-point average roughness defined in JIS-B6012, same below), but as shown in FIG. 4, there are parts of peaks and valleys on the surface of the untreated copper foil 4 (hereinafter called “rough pyramid” in this specification). Normally, the distance between peaks of rough pyramid of the copper foil is less than 5 μm. If roughening such a surface, as shown in FIG. 4, roughening particles 12 will deposit concentrated at the portions of the peaks of the rough pyramid and will not deposit much at the portions of the valleys. Further, depending on the composition of the copper plating solution and the plating conditions for the roughening treating, as shown by reference numeral 12b in FIG. 4, abnormal deposition of roughening particles will occur. Such abnormal deposition will result in so-called “residual copper” after producing and etching the copper-clad laminate and make formation of fine patterns impossible.
Further, the shiny side is relatively smooth, so experiments have been made with depositing granular copper on the shiny side to raise the bond strength with a resin substrate (for example, see Japanese Unexamined Patent Publication (Kokai) No. 6-270331). However, the shiny side of the untreated copper foil appears shiny and smooth at first glance, but as explained above, this is the surface in contact with the titanium drum, so becomes an exact replica of the titanium drum. Therefore, it is affected by surface scratches of the titanium drum and deep scratch-like defects sometimes are seen. If roughening treating such a defect surface, as shown in FIG. 5, the Rz value of the relief shapes will indeed be small, but abnormal deposition 12b of roughening particles will occur at the scratch portions, the abnormally deposited portions will turn into residual copper at the time of preparing the fine patterns, and fabrication of fine patterns will become difficult.
With the copper foil and method of treating the rough side according to the technology disclosed in the above prior art, it is not possible to cope with the demands for increasingly fine patterns of recent years when electronic equipment has been made smaller and higher in performance and when printed circuits are being required to be made smaller and higher in density. Problems such as insufficient bonding strength with the resin substrate, residual copper when forming the fine patterns, and erosion at the bottom portions of the circuit lines are being pointed out.